The invention relates to video signal compression.
In an important example, the invention concerns the MPEG-2 video signal compression standard, ISO/IEC 13818-2, though it can be applied to any video compression system that is liable to degradation when coding and decoding are cascaded.
There has already been disclosed (WO-A-9535628) the use of a signal which accompanies an MPEG bitstream and which carries information about the bitstream for use in a downstream process, for example, the re-encoding of a decoded MPEG picture. This signal is provided in parallel and is sent along an appropriate side channel to accompany a decompressed signal from a compression decoder to a subsequent encoder.
Where equipment has been specifically designed for use with such a signal, considerable advantage can be gained and many of the problems previously associated with cascaded coding and decoding processes are removed or ameliorated by using in a downstream coding process, key information concerning upstream coding and decoding.
It is an object of the present invention to extend these advantages, in part or in whole, to arrangements which include equipment not specifically designed for use with such a signal.
Accordingly, the present invention consists in one aspect in a video signal process wherein a compression encoded signal is compression decoded for passage along a video pathway, characterised in that the compression encoded signal or a portion thereof, accompanies the video signal along the video pathway.
Suitably, said portion of the compression encoded signal comprises a compressed form of the information bus as detailed in the above mentioned WO-A-9535628. This Information Bus may include the following information: picture dimensions; frame rate; picture structure (frame-coded or field-coded); picture type (I,P or B); whether macroblocks are intra-coded or use prediction; whether forward, backward or bi-directional prediction is used; motion vectors; quantiser visibility weighting matrices; quantiser step and buffer state of a downstream decoder.
In a further aspect, the present invention consists in the transport along a video pathway of a signal, containing all or part of an MPEG bitstream, alongside a video signal decoded from the MPEG bitstream.
In one form of the invention, said signal is MPEG compliant.
The advantage of such an arrangement is that bitstream information can pass transparently through studio processing equipment to facilitate re-encoding of the output to MPEG format without impairments due to cascading. For the purposes of this specification, the new signal is called the Information Stream.